Watch: Trump pitches his border wall to the nation while the government shutdown drags on

[UPDATE: Trump makes a grisly case for his border wall, describing horrifying alleged murders]

President Donald Trump was set Tuesday night to deliver his first-ever televised Oval Office address.

The speech comes as a large chunk of the federal government remains shut down as Trump demands funds to build a wall along the U.S.-Mexico border. The president has claimed there is an illegal-immigration crisis at the border, despite evidence to the contrary.

By Tuesday, the shutdown of about 25 percent of the government had stretched out to 18 days, making it the second-longest on record.

Trump has refused to accept any funding agreement that excludes billions of dollars toward the construction of a border wall, calling the situation at the border a "Humanitarian and National Security crisis."

House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer were set to respond to Trump from the U.S. Capitol following his address. The Democratic leaders have taken a firm stance against including any money for a southern border barrier as part of a deal to reopen nine federal agencies that have been affected by a partial government shutdown.

Trump had initially said he would be "proud" to shut down the government in pursuit of a wall, which he has described as a necessity for protecting the very existence of the country.

But he has since shifted to blame the gridlock on Democrats, many of whom have in turn labeled the wall a "vanity project" for the president.

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